In recent years, there has been suggested a method for obtaining a distribution of optical characteristic values in vivo at high resolution by using a characteristic of ultrasound, which is less likely to scatter in vivo compared to light.
According to PTL, a living body is irradiated with pulsed light generated by a fight source, an acoustic wave that is generated from biological tissue through energy absorption of the pulsed light is detected, and a signal corresponding to the detected acoustic wave is analyzed, whereby a distribution of optical characteristic values in vivo is obtained. The imaging using an acoustic wave obtained through irradiation of a living body with light is generally called photoacoustic imaging.
Regarding a photoacoustic imaging method, the following technique is known. That is, when a photoacoustic wave generated by irradiating a spherical optical absorber 10 with light is detected by an acoustic wave detector 20 as illustrated in FIG. 9A, N-shape acoustic pressure information illustrated in FIG. 9B is obtained if optical absorption of the optical absorber 10 is even (sec NPL).
The value obtained by multiplying a time width of the N-shape waveform by sonic speed is a value in which the size of the optical absorber 10 (here, the diameter of the sphere) is reflected. Also, the time when the N-shape waveform is detected reflects position information of the optical absorber 10. Furthermore, in a case where the intensity of light that arrives at the optical absorber 10 is equal, the magnitude of a signal having the N-shape waveform is proportional to the absorption coefficient of the optical absorber 10.
As described above, in photoacoustic imaging, an image of optical absorber 10 is reconstructed by using data obtained from a photoacoustic wave.
In the above-described photoacoustic imaging performed by detecting ultrasound that is generated through optical absorption, a tissue having an optical absorption co-efficient higher than that of a medium around an optical absorber is imaged. For example, a blood vessel in a living body has an optical absorption coefficient higher than that of a surrounding medium. Imaging of a blood vessel has been studied.
As a method for processing a photoacoustic wave signal obtained from a detector, a waveform process such as envelope detection can be used. By performing an image formation process after the waveform process, all in vivo optical characteristic distribution can be imaged.
When photoacoustic wave signals measured by detectors provided at various positions are used, an in vivo optical characteristic distribution can be imaged by using image reconstruction using a method involving an aperture synthesis process, such as delay and sum, and a waveform process, such as envelope detection.
In the near-infrared region of about 700 to 1100 nm that is used in photoacoustic imaging, there is a tissue having an optical absorption coefficient higher than that of a surrounding tissue, such as a blood vessel, and also there is a tissue having an optical absorption coefficient lower than that of a surrounding tissue, such as a calcified substance.
For this reason, in an image formation method simply using envelope detection described above, the difference in optical absorption coefficient between a subject and a surrounding medium can be detected only in the form of an absolute value. Accordingly, it has been difficult to determine whether the optical absorption coefficient of the subject is higher or lower than that of the surrounding medium.